1. Field of the Invention
The present invention relates to a method of manufacturing a sintered compact by sintering a green body, in particular an injection-molded green body, composed of titanium or titanium alloy powder.
2. Description of the Prior Art
Titanium and titanium alloys are metallic materials that are lightweight, possess high strength, exhibit excellent corrosion resistance, and have other advantages. A drawback of these materials, however, is that they have poor workability, so that they can only be used in a limited number of fields and products.
Such titanium and titanium alloys are generally cast, forged, machined, and otherwise processed to be made into finished products, but the fabrication processes are complicated and the manufacturing costs are high because laser treatments must be performed or the materials must be machined using special tools. In particular, processing for obtaining complicated and intricate shapes requires complex fabrication processes and sophisticated techniques, thereby resulting in considerably higher manufacturing costs.
In order to solve such problems, a method has been proposed for forming (compacting) titanium or titanium alloy powders to a prescribed shape and then sintering the resulting green compact in a sintering furnace to manufacture titanium or titanium alloy sintered compacts (Japanese Laid-Open Patent Application No. 6-330105).
In this method, the green compact composed of a titanium or titanium alloy powder is placed on a supporting plate (setter), and such a green compact is sintered under the condition that the green compact is placed within a case which is formed of a metal such as molybdenum or tungsten, or a ceramic such as alumina. The supporting plate is formed of alumina or another material that remains stable at high temperatures.
However, the method described above has the following drawbacks which are resulted from use of the aforementioned materials for the supporting plate and the case.
First, the supporting plate formed of alumina reacts with the titanium or titanium alloy of the green compact during sintering, thus leading to increasing the oxygen content of the resulting sintered compact. As a result, there are drawbacks that the sintered compact is brittle and its strength is lowered.
Second, the supporting plate that has already been used can be reused in the subsequent sintering step, but if the reaction product from the previous sintering step has deposited on the surface of the supporting plate, there is a drawback that this reaction product forms a partial bond with the sintered compact, which resulting in adverse affects to the surface properties of the sintered compact or lowering the dimensional accuracy (stability for shape and dimension) of the sintered compact as a result of variations in the coefficient of contraction during sintering.
A particular advantage is that a sintered compact having a complex and intricate shape can be manufactured with high dimensional accuracy when the green body is manufactured by an injection molding of a metal powder, but this process is still seriously flawed in that this advantage cannot be fully demonstrated due to the existence of the aforementioned drawbacks.
Using a fresh supporting plate for each sintering cycle can be adopted in order to resolve these problems, but this approach involves a problem that it entails higher manufacturing costs.
Further, there is another problem in that cases formed of metals or ceramics are difficult to fabricate or machine. In particular, metal materials such as titanium, molybdenum, and tungsten are scarce and expensive, and cases formed of these materials must be often replaced because they lack durability and can be used only a few times.
In addition, sintering is sometimes performed by placing getters composed of titanium or the like into the case together with green bodies. In connection with this, there is a problem that since the getters used must have a weight that reaches or exceeds 50% of the weight of the green bodies, expensive getters are consumed in large amounts. Further, productivity is low because the packing of the getters is time-consuming and only a narrow storage space is available for the green bodies.
As stated in the above, although it is thus possible to manufacture titanium and titanium alloy sintered products, the manufacturing equipment, peripheral equipment, and other types of equipment are extremely expensive, so that a radical solution of the problems mentioned above has yet to be found.